Sheet-fed printing press for simultaneous recto-verso printing of sheets, in particular for the production of security documents

ABSTRACT

There is described a sheet-fed printing press (1000; 1000*) comprising at least two printing units (200; 200.1, 200.2; 200.1 *, 200.2*) located one after the other, each printing unit (200; 200.1, 200.2; 200.1 *, 200.2*) being adapted to carry out simultaneous recto-verso printing of the sheets (S) and including two printing cylinders (105, 106) cooperating with one another and forming a printing nip, the two printing cylinders (105, 106) each collecting ink patterns from at least two associated plate cylinders (15A, 15B, 16A, 16B) wherein the two printing cylinders (105, 106) are located one above the other such that the sheets (S) travel laterally through each printing unit (200; 200.1, 200.2; 200.1 *, 200.2*) from a first lateral side (201 a; 201 a*) located upstream of the printing nip to a second lateral side (201 b; 201 b*) located downstream of the printing nip, wherein a number of at least two sheet transfer elements (110, 120, 95) is provided downstream of the printing nip of a first one (200.1; 200.1 *) and upstream of the printing nip of a second one (200.2; 200.2*) of the at least two printing units (200.1, 200.2; 200.1 *, 200.2*) to transfer the sheets (S).

TECHNICAL FIELD

The present invention generally relates to a sheet-fed printing pressadapted to carry out recto-verso printing of individual sheets, inparticular for the production of security documents such as banknotes,comprising one or more printing units each adapted to carry outsimultaneous recto-verso printing of the sheets, each printing unitincluding two printing cylinders cooperating with one another andforming a printing nip where both sides of the sheets are simultaneouslyprinted, the two printing cylinders each collecting ink patterns from atleast two associated plate cylinders. The invention especially relatesto a printing press preferably comprising a printing unit with at leasta printing group designed to print a first side and/or a second side ofthe substrate by collecting different impressions or ink patterns intheir respective colours on a cylinder, e. g. a collecting cylinder, inadvance before being printed onto the substrate.

BACKGROUND OF THE INVENTION

Such printing presses are known as such in the art, in particular fromEuropean Patent Publication No. EP 0 949 069 A1 and International PCTPublications Nos. WO 2007/042919 A2, WO 2007/105059 A1, WO 2007/105061A1, WO 2008/099330 A2, WO 2014/056711 A1, WO 2016/042482 A2 and WO2016/071870 A1, which publications are all incorporated herein byreference in their entirety. International PCT Publication No. WO2007/042919 A2 in particular discloses a recto-verso offset printingpress adapted to carry out simultaneous recto-verso printing of sheetsthat further comprises an additional printing group placed upstream of amain printing group of the printing press.

FIGS. 1 and 2 illustrate such a recto-verso printing press that isadapted to carry out simultaneous recto-verso printing of sheets S, astypically used for the production of banknotes and like securitydocuments, which printing press is designated globally by referencenumeral 100. Such printing press is in particular marketed by thepresent Applicant under the product designation Super Simultan® IV. Thebasic configuration of the printing press 100 shown in FIGS. 1 and 2 issimilar to that shown and discussed with reference to FIG. 1 ofInternational PCT Publication No. WO 2007/042919 A2. This type ofprinting press is typically designated as a so-called “Simultan” (or“Simultan-offset”) press owing to the fact that multiple colours arecollected on a common blanket before being simultaneously transferred tothe sheets S.

This printing press 100 comprises a printing unit 2, which isspecifically adapted to perform simultaneous recto-verso printing of thesheets S (according to the so-called Simultan-offset printing principle)and comprises, as is typical in the art, two blanket cylinders (orprinting cylinders) 5, 6 rotating in the directions indicated by thearrows and between which the sheets S are fed to receive multicolourimpressions on both sides, namely on the recto and verso sides. In thisexample, blanket cylinders 5, 6 are three-segment cylinders which aresupported between a pair of side frames designated by reference numeral20. The blanket cylinders 5, 6 receive and collect different inkpatterns in their respective colours from plate cylinders 15 and 16(four on each side) which are distributed around a portion of thecircumference of the blanket cylinders 5, 6. These plate cylinders 15and 16, which each carry a corresponding printing plate, are themselvesinked by corresponding inking apparatuses 25 and 26, respectively. Thetwo groups of inking apparatuses 25, 26 are advantageously supported intwo inking carriages 21, 22 that can be moved toward or away from thecentrally-located plate cylinders 15, 16 and blanket cylinders 5, 6.

As is known in the art, each printing plate is wrapped around thecorresponding plate cylinder 15, 16 and clamped at its leading end andtrailing end by a suitable plate clamping system, which plate clampingsystem is located in a corresponding cylinder pit of the plate cylinder(see e.g. International (PCT) Publications Nos. WO 2013/001518 A1, WO2013/001009 A1 and WO 2013/001010 A2, which are also incorporated hereinby reference in their entirety).

Sheets S are fed from a sheet feeder 1 onto a feeder table 1* locatednext to the printing unit 2 (on the right-hand side in FIGS. 1 and 2) toa succession of transfer cylinders 9, 8′, 10 (three cylinders in thisexample) placed upstream of the blanket cylinders 5, 6. While beingtransported by the transfer cylinder 8′, the sheets S receive a firstimpression on a first side of the sheets S (namely the upper—orrecto—side in the illustrated example, which side is identified by ablack triangle in the drawings by opposition to the lower—or verso—side,which is identified by a white triangle) using an additional printinggroup, the transfer cylinder 8′ fulfilling the additional function ofimpression cylinder. This additional printing group consists of, inaddition to the transfer cylinder 8′, a blanket cylinder 8 (atwo-segment cylinder in this example) that collects inks from two platecylinders 18 that are inked by corresponding inking apparatuses 28. Theinking apparatuses 28 are advantageously supported in an inking carriage24 that can be moved toward or away from the plate cylinders 18 andblanket cylinder 8. The sheets S that are printed by means of theadditional printing group are first dried/cured by a drying/curing unit(designated by reference numeral 50 in FIG. 2) while being transportedby the sheet transfer cylinder 8′ before being transferred to thedownstream-located main printing group. This drying/curing unit 50 canin particular be a UV curing device, such as a UV-LED curing device.

In the example of FIGS. 1 and 2, the sheets S are transferred onto thesurface of blanket cylinder 5 where a leading edge of each sheet is heldby appropriate gripper means located in cylinder pits between eachsegment of the blanket cylinder 5. Each sheet is thus transported by theblanket cylinder 5 to the printing nip between the blanket cylinders 5and 6 where simultaneous recto-verso printing occurs. Once printed onboth sides, the printed sheets S are then transferred, as known in theart, to a sheet conveying system 3 (such as a chain gripper system withspaced-apart gripper bars) for delivery in a sheet delivery unit 4comprising multiple (e.g. three) delivery pile units 41, 42, 43.Reference numeral 31 in FIG. 2 designates a pair of chain wheels locatedat the upstream end of the sheet conveying system 3.

In the example of FIGS. 1 and 2, first and second transfer cylinders ordrums 11, 12, such as suction drums or cylinders, are interposed betweenthe sheet conveying system 3 and the blanket cylinder 5. These first andsecond transfer cylinders 11, 12 are optional (and could therefore beomitted) and are designed to carry out inspection of the sheets S on therecto and verso sides as described for instance in Internationalapplication No. WO 2007/105059 A1. Reference numerals 61, 62 in FIG. 2designate corresponding inspection cameras (such as line-scan cameras)that cooperate with cylinder or drums 11, 12.

FIG. 3 schematically shows a partial side view of a printing unit,designated by reference numeral 2*, of a printing press 100* inaccordance with FIG. 3 of International PCT Publication No. WO2016/071870 A1.

The printing press 100* comprises a main printing group consisting ofelements 5, 6, 15, 16, 25, 26, including first and second printingcylinders 5, 6 cooperating with one another to form a first printing nipbetween the first and second printing cylinders 5, 6 where first andsecond sides of the sheets S are simultaneously printed, the firstprinting cylinder 5 acting as a sheet conveying cylinder of the mainprinting group. The configuration of the main printing group is as suchidentical to that of the main printing group illustrated in FIGS. 1 and2. In this other example, printing cylinders 5, 6 are likewisethree-segment cylinders which are supported between a pair of sideframes 20. The printing cylinders 5, 6 receive and collect different inkpatterns in their respective colours from first and second sets of fourplate cylinders 15, respectively 16, which are distributed around aportion of the circumference of the printing cylinders 5, 6. These platecylinders 15 and 16, which each carry a corresponding printing plate,are again inked by corresponding sets of four inking apparatuses 25 and26, respectively. The two sets of inking apparatuses 25, 26 are likewisesupported in two retractable inking carriages 21, 22 that can be movedtoward or away from the centrally-located plate cylinders 15, 16 andprinting cylinders 5, 6.

In contrast to the configuration illustrated in FIGS. 1 and 2, theadditional printing group comprises third and fourth printing cylinders7, 8 cooperating with one another to form a second printing nip betweenthe third and fourth printing cylinders 7, 8 where both sides of thesheets S are simultaneously printed, the third printing cylinder 7acting as a sheet conveying cylinder of the additional printing group.Each printing cylinder 7, 8 collects inks from corresponding sets of twoplate cylinders 17, respectively 18, that are inked by correspondinginking apparatuses 27, 28. These two sets of inking apparatuses 27, 28are likewise supported in two retractable inking carriages 23, 24 thatcan be moved toward or away from the centrally-located plate cylinders17, 18 and printing cylinders 7, 8 (which carriages 23, 24 could bedistinct from or form an integral part of the inking carriages 21, 22 ofthe main printing group).

As shown in FIG. 3, the additional printing group 7, 8, 17, 18, 27, 28is placed upstream of and above the main printing group 5, 6, 15, 16,25, 26, the first and second printing cylinders 5, 6, on the one hand,and the third and fourth printing cylinders 7, 8, on the other hand,being aligned along two horizontal planes.

The main printing group 5, 6, 15, 16, 25, 26 and the additional printinggroup 7, 8, 17, 18, 27, 28 are coupled to one another by means of anintermediate sheet conveying system comprising, in the illustratedembodiment, first to third sheet transfer cylinders 10′, 10″, 10′″interposed between the first and third printing cylinders 5, 7. Moreprecisely, the sheets S printed in the additional printing group 7, 8,17, 18, 27, 28 are transferred from the third printing cylinder 7 insuccession to the first sheet transfer cylinder 10′, to the second sheettransfer cylinder 10″, to the third sheet transfer cylinder 10′″, andthen to the first printing cylinder 5 of the main printing group.

On their way to the main printing group 5, 6, 15, 16, 25, 26, the sheetsS are dried/cured by first and second drying/curing devices 51, 52cooperating for instance respectively with the first and second sheettransfer cylinders 10′, 10″. The drying/curing devices 51, 52 aretypically UV curing devices, in particular UV-LED curing devices.

The sheets S to be printed are fed in succession from the sheet feeder(not shown in FIG. 3) onto the feeder table 1* where they areconventionally aligned before being fed to a succession of e.g. threesheet transfer cylinders 9, 9′, 9″ at the infeed. As illustrated in FIG.3, the sheets S are fed in succession by the sheet transfer cylinders 9,9′, 9″ to the third printing cylinder 7. The sheets S thus receive firstand second impressions on both sides which are performed simultaneouslyat the printing nip between the third and fourth printing cylinders 7, 8of the additional printing group and at the printing nip between thefirst and second printing cylinders 5, 6 of the main printing group.

Once fully printed, the sheets S can likewise be inspected on both sidesby means of an inspection system 11, 12, 61, 62 similar to the onedescribed with reference to FIGS. 1 and 2, and then transferred to thesheet conveying system 3 for delivery to the sheet delivery unit (notshown in FIG. 3).

In the examples of FIGS. 1 to 3, one will appreciate that the sheet pathruns through the printing unit 2, respectively 2*, from top to bottom.One limitation of the printing presses of FIGS. 1 to 3, resides in thatexpansion possibilities are somewhat limited by the availableinstallation height in the relevant printing works where the printingpress is to be installed. The basic configuration of the printingpresses of FIGS. 1 to 3 is however a huge advantage in terms ofcolour-to-colour register accuracy thanks to the fact that such printingpresses are designed to operate according to the Simultan-offsetprinting process.

FIG. 4 shows another known sheet-fed printing press, designated globallyby reference numeral 100**, for recto-verso printing press of sheets Sas for instance disclosed in U.S. Pat. No. 5,555,804, which otherpublication is also incorporated herein by reference in its entirety.This type of printing press, which (like the printing presses of FIGS. 1to 3) does not require any sheet reversal system to print both sides ofthe sheets S, is typically designated as a so-called “double-decker”press. Other examples of such double-decker presses are disclosed forinstance in Japanese Patent Publication No. JP 2004-034641 A andEuropean Patent Publications Nos. EP 0 906 826 A2, EP 0 976 555 A1, EP 1060 883 A1, EP 1 323 529 A1, EP 2 357 083 A1, EP 2 484 523 A1, EP 2 583828 A1, EP 2 647 505 A1, EP 2 653 309 A1, EP 2 756 952 A1 and EP 2 845728 A2.

In contrast to the examples of FIGS. 1 to 3, the printing press of FIG.4 is not adapted for simultaneous recto-verso printing of the sheets S.Rather, the sheets S are fed in succession from the sheet feeder 1 (viathe feeder table 1* and sheet transfer cylinder 9 at the infeed) througha series of printing units 2.1-2.8 which are designed to print the rectoand verso sides of the sheets S in an alternate and consecutive manner.More precisely, printing units 2.1, 2.3, 2.5 and 2.7 are designed toprint the upper (recto) side of the sheets S, while printing units 2.2,2.4, 2.6 and 2.8 are designed to print the lower (verso) side of thesheets S.

As depicted in FIG. 4, the printing units 2.1, 2.3, 2.5, 2.7 eachcomprise an impression cylinder 10.1-10.4 (located below the path of thesheets S) cooperating with an associated blanket cylinder 5.1-5.4(located above the path of the sheets S), which blanket cylinderreceives a relevant ink pattern to be transferred onto the recto side ofthe sheets S from a corresponding plate cylinder 15.1-15.4 inked by aninking apparatus 25.1-25.4. Once printed by the printing units 2.1, 2.3,2.5, 2.7, the sheets S are transferred from the relevant impressioncylinder 10.1-10.4 to the relevant impression cylinder 11.1-11.4 of thedownstream-located printing unit 2.2, 2.4, 2.6, 2.8, which impressioncylinder 11.1-11.4 is located above the path of the sheets S. A seriesof eight impression cylinders 10.1-10.4, 11.1-11.4 is thus provided totransport the sheets S consecutively through the eight successiveprinting units 2.1 to 2.8.

Printing units 2.2, 2.4, 2.6, 2.8 are basically the mirror image of theprinting units 2.1, 2.3, 2.5, 2.7 and likewise each comprise a blanketcylinder 6.1-6.4 (located in this case below the path of the sheets S)cooperating with the associated impression cylinder 11 locatedthereabove and receiving a relevant ink pattern to be transferred ontothe verso side of the sheets S from a corresponding plate cylinder16.1-16.4 inked by an inking apparatus 26.1-26.4.

Once printed by the last printing unit 2.8 in the sequence, the sheets Sare transferred to a sheet conveyor system 3 via a final transfercylinder 13 for transport and delivery to the sheet delivery unit 4.Only one delivery pile unit 41 is depicted in FIG. 4, but multipledelivery pile units could be contemplated as in the example of FIG. 1.

In the example of FIG. 4, one will appreciate that the sheet path runsthrough the printing units 2.1-2.8 from right to left along asubstantially horizontal direction, rather than vertically as in theexamples of FIGS. 1 to 3. Furthermore, the sheets S are subjected in theexample of FIG. 4 to eight consecutive printing steps, one in eachprinting unit 2.1-2.8, whereas, in the examples of FIGS. 1 to 3, thesheets S are subjected to only two consecutive printing steps, namely afirst printing step in the additional printing group and a second,subsequent printing step in the main printing group. In that respect,one major limitation of the printing press type depicted in FIG. 4resides in increased sheet distortion (as each printing step adds up tothe distortion of the sheets S) which negatively impactscolour-to-colour register accuracy. Furthermore, while the printingpress of FIG. 4 is flexible in that additional printing units could beprovided, the machine footprint is necessarily increased as a result ofsuch addition.

There is therefore a need for an improved printing press configurationwhich would be flexible in terms of modularity, while retaining as muchas possible the high colour-to-colour register accuracy of theaforementioned Simultan presses.

SUMMARY OF THE INVENTION

A general aim of the invention is to improve the known printing pressesof the aforementioned type.

More precisely, an aim of the present invention is to provide such aprinting press that is adapted to carry out recto-verso printing ofindividual sheets with a high colour-to-colour register accuracy whilebeing flexible in terms of expansion possibilities.

Another aim of the present invention is to provide such a printing presswhere machine operability and accessibility are not compromised.

These aims are achieved thanks to the printing press defined in theclaims. In particular, there is provided a sheet-fed printing pressadapted to carry out recto-verso printing of individual sheets, inparticular for the production of security documents such as banknotes,comprising one or more printing units each adapted to carry outsimultaneous recto-verso printing of the sheets, each printing unitincluding two printing cylinders cooperating with one another andforming a printing nip where both sides of the sheets are simultaneouslyprinted, the two printing cylinders each collecting ink patterns from atleast two associated plate cylinders. According to the invention, thetwo printing cylinders are located one above the other such that thesheets travel laterally through each printing unit from a first lateralside located upstream of the printing nip to a second lateral sideopposite to the first lateral side and located downstream of theprinting nip.

Instead or preferably in addition to the above, in a preferredembodiment a printing group of a respective printing unit is designed asprinting group for indirect printing, such as indirect lithographicprinting, i.e. offset printing, or an indirect relief printing, e. g.letterset printing, or a combination of both of them.

A so called collect printing group is designed to print at least oneside of the substrate by firstly collecting several impressions orpatterns from several plate cylinders on a printing cylinder, e. g. a socalled collecting cylinder, before being printed as a collected image asa whole onto the substrate.

The printing unit or especially the collect printing group preferablycan be configured with at least one or more inking apparatuses andassociated plate cylinders designed to enable and/or carry out offsetprinting, comprising for example a dampening system and/or at least thepossibility to place lithographic printing plates onto the respectiveplate cylinder. Although these inking apparatuses possibly can also berun for letterset printing without or with inactive dampening system andwith a letterpress printing plate, the printing group or printing unitnevertheless is designed—at least partly—as an offset printing grouprespectively printing unit. In addition to plate cylinders and inkingapparatuses designed to enable and/or carry out offset printing acollect printing group or unit can comprise additional plate cylinderswith associated inking apparatuses designed to especially carry out onlyother kinds of printing, for example letterset printing. In this sense,the above collect printing unit or group shall be understood as anoffset printing unit or group, provided at least one, more or all of itsplate cylinders and corresponding inking apparatuses is or are designedto enable and/or carry out offset printing.

In an alternative embodiment, the printing unit or especially thecollect printing group can be configured only with one or more platecylinders and associated inking apparatuses designed to enable and/orcarry out indirect relief printing, e.g. such as letterset printing.

Further advantageous embodiments of the invention form thesubject-matter of the dependent claims and are discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear moreclearly from reading the following detailed description of embodimentsof the invention which are presented solely by way of non-restrictiveexamples and illustrated by the attached drawings in which:

FIG. 1 is schematic illustration of a known Simultan-type recto-versoprinting press exhibiting a configuration similar to that disclosed inInternational PCT Publication No. WO 2007/042919 A2;

FIG. 2 is a schematic partial side view of the printing unit of theprinting press of FIG. 1;

FIG. 3 is a schematic partial side view of the printing unit of a knownSimultan-type recto-verso printing press exhibiting a configurationsimilar to that disclosed in International PCT Publication No. WO2016/071870 A1;

FIG. 4 is a schematic partial side view of a known double-decker-typerecto-verso printing press exhibiting a configuration similar to thatdisclosed in U.S. Pat. No. 5,555,804;

FIG. 5 is a schematic partial side view of a printing unit of a printingpress in accordance with a first embodiment of the invention;

FIG. 6 is a schematic partial side view of a printing press comprisingat least two printing units as shown in FIG. 5 in accordance withanother embodiment of the invention ; and

FIG. 7 is a schematic partial side view of a printing press comprisingat least two printing units in accordance with a further embodiment ofthe invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will be described in the particular context of asheet-fed recto-verso printing press adapted to carry out offsetprinting on both sides of the sheets and comprising one or more printingunits each exhibiting a (2)-over-(2) Simultan configuration. It shall beappreciated however that the invention is not limited to this particularconfiguration and could be extended to any (m)-over-(n) Simultanconfiguration where variables m and n are integers greater or equal to2. This being said, the printing press configurations as shown in FIGS.5 to 7 are particularly preferred as they exhibit a reasonably simpleconfiguration while still allowing unprecedented expansion capabilitiesand great modularity. As this will be appreciated from reading thefollowing description of embodiments of the invention, the preferred(2)-over-(2) Simultan printing unit configuration would allow anyprinting press configuration for (2×N)-over-(2×N) recto-verso printing(N being an integer number equal to the number of individual(2)-over-(2) Simultan printing units. The expression “(m)-over-(m)configuration” is to be understood as a simultaneous recto-versoprinting with m colour separations or frames printed on each side and/ora configuration of a recto-verso printing press, printing unit or groupcomprising a first set of m plate cylinders cooperating with a firstprinting cylinder and a second set of m plate cylinders cooperating witha second printing cylinder, which first and second printing cylinderscooperate to build a common printing nip.

The expression “printing group” will be used for the equipment, e.g. thecylinders, rollers and the means of the inking unit(s), belonging to aprinting nip for at least printing on one side of the substrate. Adouble sided printing group therefor is a special printing group withtwo printing groups, one on or for each side of the substrate path,sharing a same printing nip for printing simultaneously both sides of apassing substrate and mutually acting with its printing cylinders ascounter-pressure cylinders for the other printing group. It is to beunderstood, that several printing groups can be arranged in a sameprinting unit, with these printing groups for example being arranged insingle- or multi-part frame walls.

In the context of the present invention, the expression “printingcylinder(s)” will be used to designate the relevant cylinders of theprinting press that directly cooperate with the first or second side ofthe sheets S to transfer printing patterns thereon. This expression ishowever interchangeable with the expression “blanket cylinder”, it beingto be understood that the relevant printing cylinders each carry anumber of printing blankets. In the embodiments of FIGS. 5 to 7, theseprinting (or blanket) cylinders are designated by reference numerals 105and 106 and are two-segment cylinders. The printing cylinders couldhowever present any suitable size or diameter.

The expression “upper side” (or “recto side”) and “lower side” (or“verso side”) are used in the following description to designate the twoopposite sides of the sheets S being printed. More precisely, in theillustrations of FIGS. 1 to 7, the “upper/recto side” designates theside of the sheets S that is identified by the black triangles in thedrawings, while the “lower/verso side” designates the side of the sheetsS that is identified by the white triangles in the drawings. Theseexpressions are however interchangeable.

FIG. 5 schematically shows a partial side view of a printing unit,designated by reference numeral 200, of a printing press in accordancewith an embodiment of the invention. While only one printing unit 200 isdepicted in FIG. 5, the relevant printing press could comprise anydesired number of printing units 200 located one after the other, allsharing the same or substantially the same configuration. Asheet-feeder, feeder table, sheet conveying system and sheet deliveryunit are not shown in FIG. 5, but it shall be appreciated that suchcomponents—which are known as such in the art—would typically beprovided to ensure feeding of sheets S to the (first) printing unit inthe sequence and conveyance of the printed sheets S from the lastprinting unit in the sequence to the delivery unit. Furthermore, anyadditional printing or processing unit could be provided upstream ordownstream of any one of the printing units 200, including but notlimited to an inspection system for inspecting one or both sides of thesheets, a coating unit for coating one or both sides of the sheets withe.g. a varnish, or any other printing unit operating in accordance withany desired printing process (such as intaglio printing, screenprinting, letterpress printing, etc.).

The printing unit 200 illustrated in FIG. 5 is specifically designed tobe able to be coupled to at least another, identical printing unit 200,be it on the upstream side and/or downstream side. More precisely, theprinting unit 200 includes two printing cylinders 105, 106 cooperatingwith one another and forming a printing nip where both sides of thesheets S are simultaneously printed. These two printing cylinders 105,106 are located one above the other such that the sheets S travellaterally (here from the right to the left) through the printing unit200 from a first lateral side, designated by reference numeral 201 a,located upstream of the printing nip formed by the two printingcylinders 105, 106 to a second lateral side, designated by referencenumeral 201 b, opposite to the first lateral side 201 a and locateddownstream of the printing nip. In the illustrated example, the firstprinting cylinder 105 acts as a sheet conveying cylinder and transportsthe individual sheets S to and past the printing nip.

The second printing cylinder 106 could alternatively be designed as thesheet conveying cylinder, in which case conveyance of the sheets S toand away from the printing nip between the two printing cylinders 105,106 would have to be changed accordingly. In the context of the presentinvention, it does not really matter if the sheets S are conveyed by thefirst or second printing cylinder.

As illustrated in FIG. 5, the printing unit 200 is preferably designedas a (2)-over-(2) Simultan unit comprising first and second platecylinders 15A, 15B cooperating with the first printing cylinder 105 andthird and fourth plate cylinders 16A, 16B cooperating with the secondprinting cylinder 106. In the illustrated example, the first and secondplate cylinders 15A, 15B are accordingly located above the firstprinting cylinder 105, while the third and fourth plate cylinders 16A,16B are located below the second printing cylinder 106. Each of theplate cylinders 15A, 15B, 16A, 16B is inked by a corresponding inkingapparatus 25A, 25B, 26A, 26B, which inking apparatuses 25A, 25B, 26A,26B can advantageously be designed to each comprise two ink fountains,which configuration is useful for rainbow printing.

Each inking apparatus 25A, 25B, 26A, 26B is conveniently designed toextend along a substantially vertical direction above or below theassociated plate cylinder 15A, 15B, 16A, 16B, thereby freeing space toget access to the plate cylinders 15A, 15B, 16A, 16B for the purposee.g. of mounting or removing the printing plates from the circumferenceof the plate cylinders 15A, 15B, 16A, 16B. For each ink fountain, thereis defined at least one reference plane, that intersects with thatrespective ink fountain and that contains the rotational axis of theplate cylinder 15A, 15B, 16A, 16B associated with that respective inkfountain. An inking apparatus designed to extend along a substantiallyvertical direction above or below the associated plate cylinder 15A,15B, 16A, 16B is preferably characterized in that for at least one inkfountain of that inking apparatus and preferably for each ink fountainof that inking apparatus, the at least one respective reference planecontains at least one straight reference line, and in that this at leastone straight reference line is oriented perpendicularly to therotational axis of the plate cylinder 15A, 15B, 16A, 16B associated withthat respective ink fountain and in that this at least one straightreference line together with a vertical straight line confines an anglethat is smaller than 45°, more preferably smaller than 35°, morepreferably smaller than 25° and most preferably smaller than 20°.Alternatively, the inking apparatus 25A, 25B, 26A, 26B could be locatedin corresponding inking carriages designed to be retractable away fromthe plate cylinders 15A, 15B, 16A, 16B.

In the illustrated example, the first and second printing cylinders 105,106, the corresponding plate cylinders 15A, 15B, 16A, 16B, as well asthe associated inking apparatuses 25A, 25B, 26A, 26B (unless the latterare supported in inking carriages as mentioned above) are supportedbetween a pair of side frames 201, which pair of side frames 201 formscorresponding connection interfaces on the first and second lateralsides 201 a, 201 b at the input/upstream side and output/downstream sideof the printing unit 200. By way of preference, the pair of side frames201 also supports a first sheet transfer element 95, in particular afirst sheet transfer cylinder 95, cooperating with the first printingcylinder 105 upstream of the printing nip and at least a second sheettransfer element 110, in particular a second sheet transfer cylinder110, cooperating with the first printing cylinder 105 downstream of theprinting nip. Even more preferably, the pair of side frames 201 alsosupports a third sheet transfer element 120, in particular a third sheettransfer cylinder 120, cooperating with the second sheet transfercylinder 110 to take away the sheets S from the second sheet transfercylinder 110. In other words, in the example of FIG. 5, the sheets Stravel laterally through the printing unit 200 from a first sheettransfer location (or “sheet input location”) T1 located on the firstlateral side 201 a (namely the sheet transfer location to the firstsheet transfer cylinder 95) to a second sheet transfer location (or“sheet output location”) T2 located on the second lateral side 201 b(namely the sheet transfer location from the third sheet transfercylinder 120), the sheets S being transported in succession by cylinders95, 105, 110 and 120.

Sheet transfer elements 110, 120, 95 are exemplarily shown as sheettransfer cylinder 110, 120, 95 throughout the drawings. Nevertheless, atleast one of the sheet transfer elements 110, 120, 95 or a plurality ofthe sheet transfer elements 110, 120, 95 or all of the sheet transferelements 110, 120, 95 are alternatively embodied as another kind ofsheet transfer element 110, 120, 95, e.g. a chain gripper system 110,120, 95 and/or a gripper system 110, 120, 95 being moveable by atransport element, such as a cylinder or a transfer drum. In particular,at least two separate gripper systems 110, 120, 95 spaced apart in acircumferential direction around a common transfer cylinder 110, 120, 95carrying these at least two gripper systems 110, 120, 95 can be regardedas at least two transfer elements 110, 120, 95. Preferably, at least oneof the sheet transfer elements 110, 120, 95 is embodied as a grippersystem or as a chain gripper system 110, 120, 95 or as a sheet transfercylinder 110, 120, 95 comprising at least one gripper system. Morepreferably, at least two of the sheet transfer elements 110, 120, 95 areembodied as a respective gripper system and/or as a respective chaingripper system 110, 120, 95 and/or as a respective sheet transfercylinder 110, 120, 95 comprising at least one gripper system. In anexemplary embodiment, the first sheet transfer element 95 is embodied asa first sheet transfer cylinder 95 and/or the second sheet transferelement 110 is embodied as a second sheet transfer cylinder 110 and/orthe third sheet transfer element 120 is embodied as a third sheettransfer cylinder 120.

According to the preferred embodiment shown in FIG. 5, first and seconddrying or curing units 510, 520 are further located downstream of theprinting nip of the at least one and preferably of the respectiveprinting unit 200; 200.1, 2001.2; 200.1*, 200.2* between the twoprinting cylinders 105, 106, namely about a corresponding portion of thecircumference of the second and third sheet transfer cylinders 110, 120,respectively, where the sheets S are transported. The second drying orcuring unit 520 could alternatively be cooperating directly with thecircumference of the first printing cylinder 105 immediately after theprinting nip so as to dry or cure the verso/lower side of the sheets S.The drying or curing units 510, 520 are preferably UV-curing units, suchas UV-LED curing units. The second and third sheet transfer cylinders110, 120 could again alternatively be embodied as any other kind ofsheet transfer elements 110, 120.

Ideally, the sheet transfer location T1 to the first sheet transferelement 95 or first sheet transfer cylinder 95 and the sheet transferlocation T2 away from the third sheet transfer element 120 or thirdsheet transfer cylinder 120 are located at a same height, as illustratedin FIG. 5, thereby allowing a direct coupling of two identical printingunits 200 one after the other, in which case the third sheet transferelement 120 or third sheet transfer cylinder 120 of the upstream-locatedprinting unit 200 is made to cooperate directly with the first sheettransfer element 95 or first sheet transfer cylinder 95 of thedownstream-located printing unit 200.

Such a configuration where at least two identical printing units 200.1,200.2 are coupled directly to one another is shown in the embodiment ofFIG. 6. Indeed, FIG. 6 is a schematic partial side view of a printingpress designated by reference numeral 1000 and comprising at least twoprinting units 200.1, 200.2 of the same type as previously described inconnection with FIG. 5. Components 15A, 15B, 16A, 16B, 25A, 25B, 26A,26B, 95, 105, 106, 110, 120, 201, 510, 520 in FIG. 6 are the samecomponents as described in connection with FIG. 5 and do not thereforeneed to be described again. It suffices to understand that the twoprinting units 200.1, 200.2 are coupled directly to one another, withthe second lateral side 201 b of the first printing unit 200.1 beingcoupled to the first lateral side 201 a of the second printing unit200.2. In the illustration of FIG. 6, the first lateral side 201 a ofthe first printing unit 200.1 is shown coupled to a feeder table 1* of asheet feeder, which feeder table 1* is provided with a sheet transferelement 90 or sheet transfer cylinder 90 acting as infeed and receivingindividual sheets S from e.g. a swing-gripper arm system. The secondlateral side 201 b of the second printing unit 200.2 (not shown in FIG.6) could likewise be coupled to a third printing unit in a similarmanner to the first and second printing units 200.1, 200.2.

The last printing unit in the sequence of printing units could likewisebe coupled to any suitable sheet conveying system for transporting theprinted sheets S to a corresponding sheet delivery unit in a mannersimilar to what is shown e.g. in FIG. 4, in which case a suitable sheettransfer element or sheet transfer cylinder could equally be interposedbetween the third sheet transfer element 120 or third sheet transfercylinder 120 of the last printing unit in the sequence and the upstreamend of a sheet conveying system 3 (like the sheet transfer cylinder 13depicted in FIG. 4).

While this is not specifically illustrated, an inspection system similarto the inspection system 11, 12, 61, 62 shown in FIGS. 1 to 3 couldlikewise be provided downstream of the last printing unit with a view toinspect both sides of the printed sheets S.

FIG. 7 shows another embodiment of a printing press, designated byreference numeral 1000*, according to the invention and comprising atleast two printing units 200.1*, 200.2* which are located one after theother. Like in the embodiment of FIG. 6, an uneven number of sheettransfer elements, in particular sheet transfer cylinders, namelytransfer cylinders 110, 120 and 95, is provided downstream of theprinting nip of the first printing unit 200.1* and upstream of theprinting nip of the second printing unit 200.2* to transfer the sheets Sfrom the first printing unit 200.1* to the second printing unit 200.2*.The only difference with respect to the embodiment of FIG. 6 resides inthat not all of the sheet transfer elements 110, 120, 95 or sheettransfer cylinders 110, 120, 95 are an integral part of the first andsecond printing units 200.1*, 200.2*. Rather, in the example of FIG. 7,only the first sheet transfer element 95 or sheet transfer cylinder 95and the second sheet transfer element 110 or sheet transfer cylinders110 are supported between the pair of side frames 201* of the twoprinting units 200.1*, 200.2*, the third sheet transfer element 120 orsheet transfer cylinder 120 being supported in this case in anintermediate frame 205 that is interposed between the second lateralside 201 b* of the first printing unit 200.1* and the first lateral side201 a* of the second printing unit 200.2*. In the illustrated example,the second lateral side 201 b* of the second printing unit 200.2* islikewise coupled to an intermediate frame 205 supporting the third sheettransfer element 120 or third sheet transfer cylinder 120 downstream ofthe second printing unit 200.2*.

From a functional perspective, components 1*, 15A, 15B, 16A, 16B, 25A,25B, 26A, 26B, 90, 95, 105, 106, 110, 120, 201*, 205, 510, 520 in FIG. 7fulfill the same function as components 1*, 15A, 15B, 16A, 16B, 25A,25B, 26A, 26B, 90, 95, 105, 106, 110, 120, 201, 510, 520 described inconnection with FIG. 6.

Various modifications and/or improvements may be made to theabove-described embodiments without departing from the scope of theinvention as defined by the annexed claims.

First of all, any one of the two printing cylinders 105, 106 could bedesigned as a sheet conveying cylinder and the invention is thereforenot limited to the illustrated embodiments where the sheet conveyingcylinder is located above the path of the sheets S. The sheet conveyingcylinder could equally be located below the path of the sheets S. In oneembodiment, the sheet conveying cylinder of a first printing unit islocated above the path of the sheets S while a sheet conveying cylinderof an adjacent second printing unit is located below the path of thesheets S. In another embodiment, the sheet conveying cylinder of a firstprinting unit is located bellow the path of the sheets S while a sheetconveying cylinder of an adjacent second printing unit is located abovethe path of the sheets S.

Similarly, while FIGS. 6 and 7 show that three sheet transfer cylinders110, 120, 95 are provided downstream of the printing nip of the firstprinting unit 200.1, respectively 200.1*, and upstream of the printingnip of the second printing unit 200.2, respectively 200.2*, to ensuretransfer of the sheets S from one printing unit to the other, any numberof at least two sheet transfer elements or cylinders would beconvenient. The number of at least two transfer elements or cylinders ispreferably any uneven number of sheet transfer elements or cylinders andmore preferably an uneven number of at least three sheet transferelements or cylinders. The illustrated examples are advantageous in thatthe at least two sheet transfer elements 110, 120, 95 and in particularthe at least three sheet transfer elements 110, 120, 95 or cylinders110, 120, 95 ensure sufficient spacing between two successive printingunits and accessibility to all relevant parts of the printing presses,in particular the plate cylinders 15A, 15B, 16A, 16B and drying/curingunits 510, 520.

Alternatively, the three sheet transfer elements 110, 120, 95 orcylinders 110, 120, 95 could be replaced by another number, inparticular another uneven number of sheet transfer elements orcylinders, for example five sheet transfer cylinders of the size assheet transfer cylinder 95 shown in FIG. 6 and/or FIG. 7.

It shall furthermore be appreciated that all relevant printing units donot necessarily need to be strictly identical. The embodiments disclosedherein however have a major advantage in that full modularity is ensuredas each individual printing unit is designed as an elementary buildingblock that can be added or removed from the printing press without majordifficulties.

LIST OF REFERENCE NUMERALS USED THEREIN

-   100 Simultan-type printing press—(2+4)-over-(4) configuration (prior    art of FIGS. 1 and 2)-   100* Simultan-type printing press—(2+4)-over-(2+4) configuration    (prior art of FIG. 3)-   100** Double-decker-type printing press—(4)-over-(4) configuration    (prior art of FIG. 4)-   1000 printing press—(2+2)-over-(2+2) configuration (embodiment of    FIG. 6)-   1000* printing press—(2+2)-over-(2+2) configuration (embodiment of    FIG. 7)-   1 sheet feeder-   1* feeder table-   S sheets-   2 printing unit (prior art of FIGS. 1 and 2)-   2* printing unit (prior art of FIG. 3)-   2.1-2.8 printing units (prior art of FIG. 4)-   200 printing unit (embodiment of FIG. 5)-   200.1, 200.2 printing units (embodiment of FIG. 6)-   200.1*, 200.2* printing units (embodiment of FIG. 7)-   3 sheet conveying system (chain gripper system with spaced-apart    gripper bars)-   31 pair of chain wheels of sheet conveying system 3 (upstream end)-   4 sheet delivery unit-   41, 42, 43 delivery pile units-   5 sheet conveying cylinder/(first) printing cylinder (main printing    group)/three-segment blanket cylinder (FIGS. 1 to 3)-   5.1-5.4 printing cylinders of printing units 2.1, 2.3, 2.5 and    2.7/one-segment blanket cylinders (FIG. 4)-   105 sheet conveying cylinder/(first) printing cylinder/two-segment    blanket cylinder (embodiments of FIGS. 5 to 7)-   6 (second) printing cylinder (main printing group)/three-segment    blanket cylinder (FIGS. 1 to 3)-   6.1-6.4 printing cylinders of printing units 2.2, 2.4, 2.6 and    2.8/one-segment blanket cylinders (FIG. 4)-   106 (second) printing cylinder/two-segment blanket cylinder    (embodiments of FIGS. 5 to 7)-   7 sheet conveying cylinder/(third) printing cylinder (additional    printing group)/two-segment blanket cylinder (FIG. 3)-   8 (fourth) printing cylinder (additional printing group)/two-segment    blanket cylinder (FIGS. 1 to 3)-   8′ sheet conveying cylinder/two-segment cylinder (FIGS. 1 and 2)-   9 sheet transfer cylinder (infeed—FIGS. 1 to 4)-   9′, 9″ sheet transfer cylinders (FIG. 3)-   90 sheet transfer cylinder (infeed—embodiment of FIGS. 6 and 7)-   95 (first) sheet transfer cylinder (embodiments of FIGS. 5 to 7)-   10 sheet transfer cylinder (FIGS. 1 to 2)-   10′, 10″, 10″′ sheet transfer cylinders (intermediate sheet    conveying system interposed between additional printing group and    main printing group—FIG. 3)-   11 inspection cylinder or drum (part of inspection system—FIGS. 1 to    3)-   12 inspection cylinder or drum (part of inspection system—FIGS. 1 to    3)-   10.1-10.4 sheet conveying cylinders/impression cylinders of printing    units 2.1, 2.3, 2.5 and 2.7/two-segment impression cylinders (FIG.    4)-   11.1-11.4 sheet conveying cylinders/impression cylinders of printing    units 2.2, 2.4, 2.6 and 2.8/two-segment impression cylinders (FIG.    4)-   13 sheet transfer cylinder/two-segment cylinder (FIG. 4)-   110 (second) sheet transfer cylinder/two-segment cylinder    (embodiments of FIGS. 5 to 7)-   120 (third) sheet transfer cylinder/two-segment cylinder    (embodiments of FIGS. 5 to 7)-   15 (four) plate cylinders cooperating with printing cylinder 5    (lower/verso side of sheets S)/one-segment plate cylinders (FIGS. 1    to 3)-   15.1-15.4 plate cylinders of printing units 2.1, 2.3, 2.5 and 2.7    (upper/recto side of sheets S)/one-segment plate cylinders (FIG. 4)-   15A, 15B (two) plate cylinders of printing unit 200, 200.1, 200.2,    200.1*, 200.2* (upper/recto side of sheets S)/one-segment plate    cylinders (FIGS. 5 to 7)-   16 (four) plate cylinders cooperating with printing cylinder 6    (upper/recto side of sheets S)/one-segment plate cylinders (FIGS. 1    to 3)-   16.1-16.4 plate cylinders of printing units 2.2, 2.4, 2.6 and 2.8    (lower/verso side of sheets S)/one-segment plate cylinders (FIG. 4)-   16A, 16B (two) plate cylinders of printing unit 200, 200.1, 200.2,    200.1*, 200.2* (lower/verso side of sheets S)/one-segment plate    cylinders (FIGS. 5 to 7)-   17 (two) plate cylinders cooperating with printing cylinder 7    (lower/verso side of sheets S)/one-segment plate cylinders (FIG. 3)-   18 (two) plate cylinders cooperating with printing cylinder 8    (upper/recto side of sheets S)/one-segment plate cylinders (FIGS. 1    to 3)-   20 printing press main frame/pair of side frames (FIGS. 1 to 4)-   201 frame of printing unit 200, 200.1, 200.2/pair of side frames    (embodiments of FIGS. 5 and 6)-   201 a connection interface of frame 201/first lateral side    (input/upstream side)-   201 b connection interface of frame 201/second lateral side    (output/downstream side)-   201* frame of printing unit 200.1*, 200.2*/pair of side frames    (embodiment of FIG. 7)-   201 a* connecting interface of frame 201*/first lateral side    (input/upstream side)-   201 b* connecting interface of frame 201*/second lateral side    (output/downstream side)-   205 intermediate frame (FIG. 7)-   21 retractable inking carriage supporting inking apparatuses 25-   22 retractable inking carriage supporting inking apparatuses 26-   23 retractable inking carriage supporting inking apparatuses 27    (FIG. 3)-   24 retractable inking carriage supporting inking apparatuses 28    (FIGS. 1 to 3)-   25 (four) inking apparatuses each cooperating with a corresponding    one of the plate cylinders 15 (FIGS. 1 to 3)-   25.1-25.4 inking apparatus of printing units 2.1, 2.3, 2.5 and 2.7    cooperating with plate cylinder 15.1, 15.2, 15.3, 15.4, respectively    (FIG. 4)-   25A, 25B (two) inking apparatuses each cooperating with a    corresponding one of the plate cylinders 15A, 15B (FIGS. 5 to 7)-   26 (four) inking apparatuses each cooperating with a corresponding    one of the plate cylinders 16 (FIGS. 1 to 3)-   26.1-26.4 inking apparatus of printing units 2.2, 2.4, 2.6 and 2.8    cooperating with plate cylinder 16.1, 16.2, 16.3, 16.4, respectively    (FIG. 4)-   26A, 26B (two) inking apparatuses each cooperating with a    corresponding one of the plate cylinders 16A, 16B (FIGS. 5 to 7)-   27 (two) inking apparatuses each cooperating with a corresponding    one of the plate cylinders 17 (FIG. 3)-   28 (two) inking apparatuses each cooperating with a corresponding    one of the plate cylinders 18 (FIGS. 1 to 3)-   50 drying/curing unit for recto side of sheets S, e.g. UV-LED curing    unit (FIGS. 1 and 2)-   51 (first) drying/curing unit for verso side of sheets S, e.g.    UV-LED curing unit, cooperating with sheet transfer cylinder 10′    (FIG. 3)-   52 (second) drying/curing unit for recto side of sheets S, e.g.    UV-LED curing unit, cooperating with sheet transfer cylinder 10″    (FIG. 3)-   510 (first) drying/curing unit for recto side of sheets S, e.g.    UV-LED curing unit, cooperating with sheet transfer cylinder 110    (embodiments of FIGS. 5 to 7)-   520 (second) drying/curing unit for verso side of sheets S, e.g.    UV-LED curing unit, cooperating with sheet transfer cylinder 120    (embodiments of FIGS. 5 to 7)-   61 inspection camera (verso side of sheets S) cooperating with    inspection cylinder or drum 11, e.g. line-scan camera (FIGS. 1 to 3)-   62 inspection camera (recto side of sheets S) cooperating with    inspection cylinder or drum 12, e.g. line-scan camera (FIGS. 1 to 3)-   T1 sheet transfer location to (first) sheet transfer cylinder    95/sheet input location-   T2 sheet transfer location away from (third) sheet transfer cylinder    120/sheet output location

1-26. (canceled)
 27. A sheet-fed printing press adapted to carry outrecto-verso printing of individual sheets, comprising one or moreprinting units each adapted to carry out simultaneous recto-versoprinting of the sheets, each printing unit including two printingcylinders cooperating with one another and forming a printing nip whereboth sides of the sheets are simultaneously printed, a first printingcylinder of the two printing cylinders collecting ink patterns from atleast two plate cylinders associated with the first printing cylinderand a second printing cylinder of the two printing cylinders collectingink patterns from at least two plate cylinders associated with thesecond printing cylinder and wherein the two printing cylinders arelocated one above the other such that the sheets travel laterallythrough each printing unit from a first lateral side located upstream ofthe printing nip to a second lateral side opposite to the first lateralside and located downstream of the printing nip, characterized in thatthe sheet-fed printing press is comprising at least two of said printingunits that are located one after the other, wherein a number of at leasttwo sheet transfer elements is provided downstream of the printing nipof a first one of the at least two printing units and upstream of theprinting nip of a second one of the at least two printing units totransfer the sheets from the first printing unit to the second printingunit and in that the plate cylinders are each inked by an associatedinking apparatus, and in that for each ink fountain, there is defined atleast one reference plane, that intersects with that respective inkfountain and that contains the rotational axis of the plate cylinderassociated with that respective ink fountain each inking apparatus beingcharacterized in that the at least one respective reference planecontains at least one straight reference line, and in that this at leastone straight reference line is oriented perpendicularly to therotational axis of the plate cylinder associated with that respectiveink fountain and in that this at least one straight reference linetogether with a vertical straight line confines an angle that is smallerthan 35°.
 28. The sheet-fed printing press according to claim 27,wherein at least part of the number of at least two sheet transferelements is supported in an intermediate frame interposed between thesecond lateral side of the first printing unit and the first lateralside of the second printing unit.
 29. The sheet-fed printing pressaccording to claim 27, wherein the at least two printing units arecoupled directly to one another, with the second lateral side of thefirst printing unit being coupled to the first lateral side of thesecond printing unit, and wherein the number of at least two sheettransfer elements is an integral part of the first and second printingunits.
 30. The sheet-fed printing press according to claim 27, whereinthe two printing cylinders comprise a first printing cylinder acting assheet-conveying cylinder, wherein the sheets are transferred to thefirst printing cylinder by means of a first sheet transfer element orfirst sheet transfer cylinder cooperating with the first printingcylinder upstream of the printing nip, wherein the sheets are taken awayfrom the first printing cylinder by means of a second sheet transferelement or second sheet transfer cylinder cooperating with the firstprinting cylinder downstream of the printing nip.
 31. The sheet-fedprinting press according to claim 30, wherein each printing unitcomprises a pair of side frames supporting at least the two printingcylinders as well as the first sheet transfer element or first sheettransfer cylinder and/or the second sheet transfer element or secondsheet transfer cylinders.
 32. The sheet-fed printing press according toclaim 27, wherein each printing unit is configured as a—over-Simultanunit comprising first and second plate cylinders cooperating with afirst one of the two printing cylinders to print a first side of thesheets and third and fourth plate cylinders cooperating with a secondone of the two printing cylinders to print a second side of the sheets.33. The sheet-fed printing press according to claim 27, wherein theplate cylinders are each inked by an associated inking apparatus, theinking apparatuses each comprising two ink fountains.
 34. The sheet-fedprinting press according to claim 27, further comprising at least onerespective drying or curing unit located downstream of the respectiveprinting nip between the two printing cylinders of the respectiveprinting unit.
 35. The sheet-fed printing press according to claim 34,comprising first and second drying or curing units each cooperating withan associated sheet transfer element to dry first and second sides ofthe sheets.
 36. The sheet-fed printing press according to claim 35,wherein each printing unit is provided with the first and second dryingor curing units and associated sheet transfer elements.
 37. Thesheet-fed printing press according to claim 34, wherein the least onedrying or curing unit is a UV-curing unit and/or a UV-LED curing unit.38. The sheet-fed printing press according to claim 27, wherein thesheet-fed printing press is a sheet-fed printing press adapted to carryout recto-verso printing of individual sheets for the production ofsecurity documents such as banknotes.
 39. The sheet-fed printing pressaccording to claim 27, wherein this at least one straight reference linetogether with a vertical straight line confines an angle that is smallerthan 25° and/or smaller than 20°.
 40. The sheet-fed printing pressaccording to claim 27, wherein at least one of the sheet transferelements is embodied as a gripper system or as a chain gripper system oras a sheet transfer cylinder comprising at least one gripper systemand/or wherein the first sheet transfer element is embodied as a firstsheet transfer cylinder and/or the second sheet transfer element isembodied as a second sheet transfer cylinder and/or the third sheettransfer element is embodied as a third sheet transfer cylinder.
 41. Thesheet-fed printing press according to claim 27, wherein the number of atleast two sheet transfer elements is an uneven number of at least threesheet transfer elements.